1. Field of the Invention
The present invention relates to a method of forming a silicon film that is used in an electrode of a capacitor as a semiconductor device.
2. Description of the Related Art
In recent years, the size of a memory is reduced with the high integration of Dynamic Random Access Memories. This causes the area of a memory cell capacitor to be reduced, and consequently a problem arises which the capacitance of a memory cell is also reduced. Then, in order to obtain sufficient capacitance, there has been proposed a method of increasing a capacitor area while keeping an insulation film thickness in the status quo.
For example, a method described in Japanese Patent Laid-open Publication No. 4-252018 is shown in FIGS. 1A and 1B. An amorphous insulation film (oxide film) 2 is formed on a substrate 1, and then an amorphous silicon 3 is formed on the insulation film 2. Immediately after formation of the amorphous silicon 3, plasma hydrogen 4 is supplied to the surface of the amorphous silicon 3 without exposing it to the air. The surface atoms of the amorphous silicon 3 are terminated by the hydrogen atoms 5 and become inactive. Therefore, even if it is left in the atmosphere for a long period of time, formation of a natural oxide film would be prevented. Since these hydrogen atoms 5 are dissociated from silicon atoms at a lower temperature than the amorphous silicon layer 3 crystallizes, a clean surface is obtained prior to the crystallization of amorphous silicon. The diffusion speed of the silicon atoms on the clean surface is extremely fast, so that seed grains are formed and grown on the surface of the amorphous silicon 3, and as shown in FIG. 1B there is formed polycrystal silicon 7 having a lot of hemispherical polycrystal grains 6 on its surface, resulting in very large surface area. If the polycrystal silicon 7 formed in this way is used as a capacitor electrode, the capacitance is approximately twice or more as large as in a case where plasma hydrogen 4 is not supplied to the amorphous silicon 3.
However, the conventional method has practical difficulties since the clean surface of the amorphous silicon 3 is unstable and difficult to hold for a long period of time. Moreover, heat treatment under very high vacuum conditions is needed for grain formation and growth. Therefore, the equipment for forming the polycrystal silicon 7 is expensive, and temperature control of the heat treatment is not easy because an allowable range of heat treatment temperature is narrow.